This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The overall theme guiding this and future work is to better understand the interactions between age, physical activity, and obesity on mechanisms that regulate insulin action. Insulin resistance, diabetes, obesity, and cardiometabolic syndrome are closely related health problems that are present in ever-growing numbers of adults and children. It is well established that improvement of these conditions can be achieved through lifestyle modification, but the impact of specific programs may vary with the approach used and the target population. In this proposal we will determine whether the response of insulin action to aerobic exercise training varies with age and body fatness. Our goal is to compare exercise responses in children and adults. Thus, a scientifically valuable, yet practical outcome measure of insulin action is needed, particularly for children. In Specific Aim 1 we will establish the validity, reproducibility, and sensitivity of a minimally invasive method for assessing meal glucose tolerance in children and young adults. A mixed meal containing carbohydrate, protein, and fat will be tested, making it more physiologically relevant than the standard oral glucose tolerance test, but less invasive than either the insulin clamp or intravenous glucose tolerance test (IVGTT). The results will be tested against the IVGTT in normal-weight and overweight/obese subjects. The validity and reproducibility of insulin sensitivity determined from the meal test in children has not been reported so this study is needed to evaluate this test for future studies. In Specific Aim 2 we will compare the effects of a 16-week program of aerobic exercise training, and short-term detraining, on insulin sensitivity among normal weight and overweight children, and normal weight and obese young and older adults. This aim builds upon recent findings that the response of insulin sensitivity to exercise may decline with age. Normal and overweight/obese subjects will remain weight-stable throughout the training program and will undergo testing at 1, 5 and 14-days after the training program. This will determine the acute effect of training on insulin action, the durability of the training effect, and the potential interactions of age and body fatness on insulin action. The outcomes of the proposed studies will increase our ability to obtain practical and meaningful measures of insulin action in children and adults, and will help determine whether specific exercise recommendations are needed for people based on age and body fatness.